The present invention relates to a process for production of polypropylene sheets or films which are of high transparency although not stretched and thus which are suitable for use in wrapping of foodstuffs, medicines and so forth.
Sheets or films made of crystalline thermoplastic resins, such as polypropylene, are varied in physical properties such as transparency depending on the crystal form thereof.
In order to improve transparency of crystalline thermoplastic resin sheets or films, various techniques for controlling the crystallization by rapid cooling (Japanese Patent Publication Nos. 41457/1987 and 10816/1987, Japanese Patent Application Laid-Open Nos. 42822/1987 and 236719/1985, etc.), have been proposed and have been put into practical use.
In recent years, sheet products have been required to have increased in quality and productivity.
In the production of sheets having a thickness of 0.2 mm, for example, in accordance with the above methods, if the film-forming speed is increased to more than 20 m/sec, particularly more than 30 m/sec, the transparency of the sheet is gradually decreased. This is considered due to the formation of shark skin in the sheet surface with an increase of die shear stress.
From a viewpoint of the increase of productivity, sheets or films free from the decrease of transparency even at a film-forming speed of more than 20 m/sec, particularly more than 30 m/sec are desirable.
In accordance with the conventional methods, when the thickness of the sheet is more than 0.6 mm, the limit of conduction of heat in the thickness direction causes a problem that the transparency is decreased abruptly.
Under such circumstances, sheets which are of high transparency although of high thickness have been desired.
In accordance with methods other than the aforementioned methods, which have heretofore been proposed, e.g., a roll method and a method in which a nucleating agent is added, the transparency itself is poor irrespective of the film-forming speed and the thickness of the sheet.